Side member structure for print head



May 30, 1 967 Filed Aug. 6, 1965 E. J. WEST SIDE MEMBER STRUCTURE FOR PRINT HEAD 4 Sheets-Sheet l BY W W May 30, 1967 E. J. WEST 3,322,252,

SIDE MEMBER STRUCTURE FOR PRIN T HEAD Filed Aug. 6, 1965 4 Sheets-Sheet 2 I N VE N TOR. fuzzzrr/ My 7m! ZONES BY 2 Q Z gm May 30, 1967 E. J. WEST 33 SIDE MEMBER STRUCTURE FOR PRINT HEAD Filed Aug. 1965 4 Shays-Sheet 7 2506 8 2M5; fZb 72 24551.

' ZZZ /61 Z2 Z50 INVENTOR. il ffi'f'f W557- May 30, 1967 E. J. WEST SIDE MEMBER STRUCTURE FOR PRINT HEAD Filed Aug. 6, 1965 INV'ENTOR. 5442;- J. fl/Zff'" BY m f ra/mv/ 4Sheets-Sheet 4 United States Patent 3,322,252 SIDE MEMBER STRUCTURE FOR PRINT HEAD Everett J. West, Collingswood, N.J., assignor to Radio Corporation of America, a corporation of Delaware Filed Aug. 6, 1965, Ser. No. 477,871 8 Claims. (Cl. 197-4) This invention relates to printers and, in particular, to an improved printer mechanism for use in a serial printer.

A serial printer is one in which characters are printed seriatim at selected, successive print locations along a document print line. One advantage of a serial printer is that it is relatively simple and inexpensive as compared to a parallel printer or an on-the-fiy printer, and requires little or no buffering or memory. An example of a serial printer is the so-called facsimile or matrix printer in which each different character is made up of a distinctive group of small picture elements.

In one known matrix printer, the picture elements of a character are printed column by column from carbon paper by the pressure between seven parallel, independently movable print bars and a scanning anvil. The print bars are located on one side of the document and carbon,

and extend across the entire width of the document. These print bars are individually controlled by electromagnets and are movable in a direction generally perpendicular to the surface of the document or paper stock. The anvil is located on the opposite side of the document or paper stock and moves at constant speed from one end of the print line to the other. A character is printed by selective energization of the electromagnets during a series of timed intervals during which the anvil is moved a distance corresponding to the width of a printed character.

In order to accomplish printing at high speed in such a printer, the print bars should be located as close to the paper as possible, whereby they are required to move only a very short distance during printing, preferably of the order of a few thousandths of an inch. At this close spacing, very large forces are required to effect printing at an acceptable and/or uniform print density if there is a cushion of air between the paper and carbon. This is especially true if multiple copies are being printed at the same time. Furthermore, if the printer bars have a fixed rest position, then the distance which the bars travel during printing varies in dependence upon the number of copies being printed, as does the density of print throughout the several copies.

Accordingly, it is one object of this invention to provide an improved printer mechanism.

It is another object of this invention to provide an improved printer mechanism having means for squeezing out the air between the several documents and carbons at the print station.

It is still another object of this invention to provide an improved printer mechanism in which the rest positions of the printer bars are self-adjusting so that the distance the printer bars move during printing is independent of the number of copies being printed.

In apparatus embodying the invention, a print head is located on one side of a document path and a backing member, e.g. a platen, is located on the other side of the path. The print head includes a plurality of elongated printer bar structures disposed in side-by-side relationship and each having a printing surface facing the document path. First and second elongated side members retain the printer bar structures in stacked array. Each side member has an elongated leg, the legs being parallel to one another and having flat, bottom faces which lie in the same plane and which face the document path.

The print head is biased toward the document path so had to FIGURES 1 and 2.

3,322,252 Patented May 30, 1967 that the faces of the legs press the documents firmly against the backing member and squeeze out the air between the documents. The printer bar structures are suspended for individual movement under their own weight into light contact with the top document.

In the accompanying drawing, like reference characters denote like components, and:

FIGURE 1 is a view in perspective of a printer system embodying the invention;

FIGURE 2 is a plan view of the printer system;

FIGURE 3 is a fragmentary view taken in the general direction of arrow A (FIGURE 2), and showing details of the printer bar structures and anvil assembly;

FIGURE 4 is an end view of the print station, taken along the lines 4-4 of FIGURE 2, but with the mounting bar, end plate, and portions of the: printer bar housing removed to show the relation of the printer bar structures to the anvil assembly;

FIGURE 5 is a side view of one of the print head mounting and positioning assemblies; and

FIGURE 6 is a diagram illustrating the printing technique.

The overall printer system will first be described in general terms, after which various portions of the system will be described in detail. Reference should first be As shown therein, the printer system is supported by and between a pair of vertical side plates 12, 14 which, in turn, are mounted on, or otherwise supported by, a horizontal base plate 10. A platen 16 extends between, and is supported by, the side plates 12, 14 in an elevated position. The top surface of platen 16 lies in a horizontal plane and has a generally oval shaped raceway 18 therein. Several rolling anvil assemblies 20, three of which are shown in FIGURE 2, move continuously along the raceway. The forward and rear sections of the raceway are straight.

Positioned above platen 16 is a print head which com prises a printer bar housing 24 containing a plurality of elongated printer bar structures. These structures extend from one end of the housing to the other and overlie the forward section of raceway 18. The printer bar structures, which are illustrated in FIGURE 3 and will be described in detail hereinafter, may be seen in FIGURE 1 through the cutout in the front of housing 24. As will be described, the individual printer bar structures are pivotally mounted for movement toward and away from platen 16. At the back right side of the printer bar housing is a solenoid block 26 which houses a plurality of solenoids for individually driving the printer bar structures. Signals are applied to the individual solenoids by way of separate leads in a cable 28.

At the left side of the printer bar housing, as viewed in FIGURE 2, is an end plate 32 to which the housing 24 is affixed. Plate 32 is pivotally pinned to a lifting bar 34 which is pivotally pinned at its forward end to a stud 36 mounted on side plate 14. The other end of bar 34 overlies a free-turning roller 40 carried by the vertical sides 42 of a supporting yoke. A similar lifting bar assembly is located at the right side of the printer bar housing and comprises a lifting bar 50 pivotally pinned at its forward end to a stud 52 mounted on side plate 12. The back end of lifting bar 50 overlies a free-turning roller 54 carried by the vertical side members 56 of a second supporting yoke. Bar 50 is pivotally pinned at 60 to an end plate 58 afiixed at the right end of the printer bar housing 24. As illustrated in FIGURE 2, end plate 58' may also serve as a support for the solenoid block 26. The aforementioned yokes are fixedly mounted on a shaft 62, which is rotatable in bearings (not shown) in side walls 12 and 14. The right hand end of the shaft projects through an aperture in side plate 12 and is coupled to the shaft 64 of a rotary solenoid 66.

Documents to be printed are movable along a feed path between the top of platen 16 and the bottom of printer bar housing 24. Thus, the printer bar structures are located above the document at the print station, and the anvil assemblies 20 move beneath the document. The anvil assemblies are so spaced from one another that only one anvil assembly moves beneath the printer bars at any one time. The document may be one having a carbon backing. Alternatively, a separate sheet of carbon may be provided for each document at the print station.

The printer system is designed to handle documents in the form of a continuous sheet of paper stock supplied, for example, from a roll or from a fanfold stack (not shown). A section of paper stock is illustrated in phantom in the drawing and may be identified by reference character 72. Multiple copies may be printed simultaneously by feeding a sheaf of interleaved carbons and paper stock through the print station from different supply rolls or stacks. The paper stock and carbons are of the type which have sprocket feed holes near both side edges.

Located at the front of the printer is a pair of paper feed tractor assemblies 76, 78. A like pair of tractor assemblies 80, 82 is located at the rear of the printer. These assemblies are alike in construction, wherefore only assembly 78 will be described in any detail. This assembly is mounted on a fixed supporting rod 84 and includes (FIGURE 1) an endless chain 86 mounted on a pair of spaced wheels 88 and 90. Wheel 88 is an idler wheel which is part of the tractor assembly proper. Wheel 90 is a drive wheel which is mounted on a tractor shaft 92. Shaft 92 extends through apertures in the side walls 12, 14 and is rotatable in bearings therein.

Each of the links in chain 86 has a projecting tab 96 which carries an upstanding sprocket tooth 98. Both the chain 86 and the sprocket teeth 98 lie in planes which are parallel to the side walls 12 and 14. A horizontal platform 100 for supporting the paper is fixedly attached to the assembly. The sheaf of paper stock and carbons rests on platform 100 with the top sprocket teeth 98 projecting upwards through the sprocket holes in the sheaf. A retaining member 102 is hinged on the tractor assembly and is adapted to clamp down loosely on top of the sheaf to assure that the sprocket holes in the sheaf of papers are maintained in cooperation with the sprocket teeth 98. The retaining member 102 is shown in an upright position in FIGURE 1 for receiving a new document. After the document is properly positioned on the tractor assemblies 76, 78, 80 and 82, the various retaining members are moved to the horizontal or paper retaining position (see assembly 76, for example) and held in that position by bias springs 104. Retaining member 102 has an elongated slot 108 through which the top sprocket teeth 98 project when the retaining member is in the horizontal position.

All the tractor assemblies are movable. Ordinarily, of course, tractor assembly 78 is clamped in a fixed position on rod 84, and drive wheel 90 is torsionally keyed to tractor shaft 92. However this entire assembly may be unclamped and slid along rod 84 and shaft 92. The other tractors are similarly movable. Thus, paper stock of various widths can be accommodated.

A first bevel gear 110 is fixedly mounted on the left end of tractor shaft 92. Cooperating therewith, and driven thereby when tractor shaft 92 rotates, is a second bevel gear 112 mounted on the forward end of a pinion shaft 114. The latter shaft is supported near its forward end in a bearing housing 116 mounted on the left end of rod 84, and is supported near its back end in a bearing housing 118 (FIGURE 2) on the left end of fixed rod 120. A

third bevel gear 122 is mounted on the back end of pinion shaft 114 and drives a fourth bevel gear 124. Fourth bevel gear 124 drives the back tractor shaft 132 by way of a spur gear differential housed in box 128. A first sun gear (not shown) of the differential is pinned to fourth bevel gear 124 and is movable therewith. The second sun gear of the differential is mounted on the left end of tractor shaft 132, and the planetary gears are contained in the spider of the differential. The spider is biased by a spring 134, connected to a pin 136 on the side wall 14, in a direction to maintain the paper under tension in the area between the front and back tractor assemblies, whereby the paper is flat at the print station. Means (not shown) may be provided to manually disengage spring 134 so that an operator may thread a new paper into place without the differential and tractors attempting to tension the paper during the threading thereof.

During the printing of a line of information, the tractor shafts 92 and 132 are stationary. It is one feature of the invention that the sheaf of documents and carbons is pressed down firmly against the platen 16 at this time and held in clamp position by the housing 24. In order to advance the paper when printing of a line thereon is complete, it is first necessary to raise the print head away from the platen. The manner in which the housing is constructed, and the manner in which it is either pressed against the paper or moved away from the paper will become clear as the discussion proceeds.

Paper advance is controlled by a rotary solenoid 150, which receives an energizing signal over line 152. The shaft 154 of the solenoid rotates through a given angle when the solenoid is energized and, in turn, rotates a link 156 to the given angle. Slot pinned at the end of link 156 is an arm 158 which is coupled at its forward end to tractor shaft 92. Coupling may be made by way of a cam clutch 160 or other clutch arrangement of a type wherein the clutch grips the tractor shaft instantaneously when the arm 158 is rotated in a first direction, by the energized solenoid, and wherein the clutch is freely rotatable in the opposite direction.

In particular, when solenoid 150 becomes energized, the back end of arm 158 is driven in a downward direction by link 156 and, in turn, rotates tractor shaft 92 a given amount in the direction indicated in FIGURE 1. Back tractor shaft is rotated synchronously in the same direction through the gear train described previously. When solenoid becomes deenergized, the back end of arm 158 .is driven in an upward direction by link 156 to the rest position. However, arm 158 does not rotate shaft 92 during its upward movement because of the action of the clutch In order to prevent movement of the paper in the opposite direction, a ratchet wheel 164 is mounted on the tractor shaft 92 and keyed to it. A pawl arm 166 is biased into engagement with the ratchet wheel and prevents rotation of the latter in a counterclockwise direction, as viewed from the right side of the printer. The paper 72 also may be moved manually any given amount in the forward direction by turning the hand knob 168 on the end of tractor shaft 92.

The printer bar structures and their relation to other components at the print station are shown in detail in FIGURES 3 and 4. Consider the forwardmost printer bar structure in FIGURE 3 by way of example. This structure 198 comprises first and second elongated members 200, 202 which extend from one end of the printer bar housing 24 (FIGURES l and 2) to the other across the width of the paper stock. A plurality of struts 204a204n are arranged in a regular or repetitive zigzag pattern along the lengths of the elongated members 200, 202, and are joined at their opposite ends to the bottom of member 200 and the top of member 202. These structs 204a-204n maintain the elongated members 200, 202 in spaced, parallel relation, whereby the structure 198 may be made very thin and light in weight while still retaining structural stability.

This is very desirable in the interests of providing a low inertia structure for high speed printing.

The various printer bar structures, seven of which are shown, are stacked in side-by-side relation one behind the other (FIGURE 4), and are held in stacked relation by the front and back sidewalls 24a and 24b of the printer bar housing. This prevents the light weight structures from wobbling or twisting as they move. Front side wall 24a has a pair of elongated bumper ribs 210 which extend the length of the housing, and back side wall 24b has a pair of similar, opposed bumper ribs 212. The ribs 210 and 212, which are in contact with the structs of the first and last printer bar structures, respectively, provide support for the stack while at the same time presenting a relatively small surface area of frictional contact with the struts of the associated structures.

The ends of all of the printer bar structures are aligned. As may be seen in FIGURE 3, the zigzag pattern of struts of adjacent printer bar structures are out-of-phase with each other by approximately a quarter of a pitch, or a quarter of a pattern section. By this arrangement, the struts of one printer bar structure are transverse to adjacent struts of next adjacent printer bar structures, whereby there is only a small area of frictional contact between struts of adjacent structures. The aforementioned and other features of the printer bar structures are discussed in greater detail in the copending application of Maurice Artzt, Ser. No. 477,875, filed concurrently herewith and assigned to the assignee of the instant invention.

Each of the printer bar structures is pivotally mounted for movement toward and away from the platen 16. Forwardmost structure 198 has a vertically projecting tab 220 at the top right end thereof. A pivot pin 222 extends through an a erture in the tab 220 and is atfixed at its other end to a crank arm 223, which is fastened to the shaft 225 of the armature 224 of a first solenoid 226. Armature 224 rotates shaft 225 when the solenoid 226 is the housing 24b, and the right end of the pin 226 may be supported in a bearing in the back wall of the solenoid housing block 26 (FIGURE 1). Structure 198 also has one or more other vertically projecting tabs spaced along the length of the member 200. One of these latter tabs 228 is illustrated in FIGURE 3 and is pivotally pinned to a link 230 which, in turn, is pivotally mounted on a pin 232. the entire printer bar structure 198 can swing through a small angle on the pivot pin 232 and shaft 225.

Elongated members 200 and 202 are parallel to one another. Also, the pivot pin 232 and shaft 225 lie in a horizontal plane, and the distance bteween pin 232 and shaft 225 is the same as that between pins 232 and 234. Accordingly, the entire bottom surface of elongated member 202, which is the printing surface, is always in a horizontal plane and parallel to the paper, regardless of the position of the printer bar structure 198. Each of the other printer bar structures is similarly mounted. Tab 238 at the right end of the second printer bar structure, for example, is pivotally mounted on a pin 240 which is atfixed at its back end to the crank arm fixed to the shaft of the armature 242 of solenoid 244.

The printer bar structures overlie a straight section of the raceway 18 (FIGURE 2) near the front of the platen 16. As may be seen in FIGURES 3 and 4, platen 16 may comprise a lower plate 16a and an upper plate 16b sandwiched togeher. Raceway 18 is located in the top surface of upper plate 16b and comprises a generally oval shaped channel (FIGURE 2) which has a rectangular cross section (FIGURE 4), and which has a through slot at the bottom thereof. Bottom plate 16a has an oval shaped hollow beneath the area defined by the raceway. Located in this hollow, near one end of the raceway, is a drive sprocket wheel 250 (FIGURE 4). A second, idler sprocket wheel is located in the hollow near the other end of the raceway, and a chain 252 is carried by the drive and idler wheels.

Chain 252 lies directly beneath the slot in the raceway. The various anvil assemblies 20 (FIGURE 2) are coupled to different ones of the chains links. Drive wheel 250 is rotated at constant speed by a motor (not shown) and, in turn, drives the chain at constant speed. Thus, the

6 anvil assemblies are driven along the raceway at constant speed.

Each of the anvil assemblies preferably is of the general type described and illustrated in the copending application of Charles J. Young, Ser. No. 477,789, filed concurrently herewith and assigned to the assignee of the instant invention. With reference to FIGURES 3 and 4 hereof, each anvil assembly may comprise a carriage body 260 borne on front and back rollers. The back roller may take the form of a pair of wheels 262a, 262b, on a common axle, and the front roller may take the form of a pair of wheels 264a, 26412 on a second, parallel axle.

A cylindrical anvil pin 268 is cradled in rounded notches in the side walls of the carriage body 270 and projects outwardly from those walls. The locations and sizes of the notches are such that the anvil pin is in contact with all of the wheels and is free to rotate, whereby there is only a rolling action between the anvil pin 268 and paper 72, and between the anvil pin and the carriage wheels. A vertical pin 270 projects out the bottom of the carriage body 260 and through the slot at the bottom of the raceway. This pin 270 is connected to one of the links of the drive chain, and is of such length that the carriage wheels 262a, 264a and 262b, 264b roll along the lips 274 and 276 at the bottom of the channel.

As may be seen in FIGURE 4, the width of the channel is only slightly greater than the length of the anvil pin 268, thereby preventing the pin from slipping out of the carriage from either side thereof. Also, the length of the anvil pin 268 is greater than the width, or thickness, of the array of stacked printer bar structures, whereby some portion of the anvil pin is always disposed opposite to the printing surface of each printer bar. Preferably, the top surface of the anvil pin is in rolling contact with the bottom sheet in the sheaf throughout its travel from one end of the print head to the other.

Printing of any desired character at a selected location in the document print line is accomplished by the selective energization of the printer solenoids 226, 244 etc. during a series of timed intervals during which the anvil pin 268 moves beneath that selected location in the line. For example, the motion of the scanning anvil may be divided into seven equal time zones per character (FIG- URE 6). Printing may take place during any or all of five of the time Zones; the other two time zones are alloted for intercharacter spacing. To print a capital E, for example, the solenoids for all of the printer bar structures are energized during the first time zone. All of the printer bar structures then are moved in a downward direction to press the paper and carbon against the anvil pin. The result is a column of seven marks printed on the paper (FIGURE 6). At the end of the first time zone, the energizing voltage is removed from the solenoids associated with the second, third, fifth and sixth printer bar structures. The other solenoids remain energized during the second, third and fourth time zones, after which the energizing voltage is removed from the solenoid associated with the fourth, or middle solenoid. The first and last solenoids become de-energized after the fifth time zone.

In the interest of high speed printing, it is desirable that the printing surfaces of the printer bars be located as close to the paper as possible. In this way, the printer bars need move only a very short distance, and the cycle time for a printer bar may be very short. At this close spacing, however, very large forces are required of the printer solenoids to effect acceptable and uniform density of print if there is a cushion of air between the paper and carbon, especially if several copies are being printed. Moreover, if the printer bars have fixed rest positions, the distance which the bars move during printing, and hence the print density, will vary in dependence upon the number of papers and carbons in the sheaf. In the latter event, it will be necessary to make adjustments in the printer when the number of copies being printed is changed.

The aforementioned requirements are obviated in the present printer system through the use of my invention, which will now be described. As may best be seen in FIGURE 4, each of the side members 24a, 24b of the printer bar housing is terminated at the bottom thereof by an elongated leg 286a, 28%, respectively. These legs are as long as the print line on the document, and preferably extend from one end of the housing 24 to the other. The bottom surfaces of the legs 280a, 28% are flat and lie in the same plane. Further, the legs are spaced apart a distance slightly greater than the width of the channel in the raceway, whereby the bottom faces of the legs are disposed opposite solid surface portions of the upper platen plate 16b. Accordingly, these legs 280a, 28Gb may clamp down on the sheaf 72 and press the sheaf firmly against the platen 16b to squeeze out the air between the documents and carbons at the print station. This action of the legs is enhanced by the spring 134 bias on the differential 128 (FIGURE 2), which operates to maintain the documents flat and taut at the printing station.

Preferably, the force applied to the housing 24 to clamp the papers and carbons is applied at a point midway between the legs 28011, 28%. In any event, it is desirable that the applied force be evenly distributed over the entire clamping area beneath the legs. For this reason, the housing 24 should have at least a slight freedom of movement to permit self equalization of the clamping forces. Further, it must be possible to raise the print head to remove the clamping forces and allow the paper to be advanced after the printing of a line is complete. The mannet in which these objectives are achieved may be seen from a consideration of FIGURE 5, which is a view in side elevation of the right hand lifting bar assembly, end plate, etc. of FIGURE 2.

As mentioned previously, in connection with FIGURE 2, lifting bar 50 is pivotally pinned at one end to a fixed stud 52. The other end of bar 50 overlies a free-turning roller 54 carried by the vertical side members 62 of a supporting yoke. That end of bar 50 has a chamfered bottom edge. The bar may be raised (rotated counterclockwise) by applying a signal to rotary solenoid 66 (FIGURE 2) to rotate shaft 62 through a given angle. Roller 54 then rides along the chamfered nose of bar 50 and drives the bar in an upward direction, counter-clockwise about stud 52. A low rate spring 290 serves to bias bar 50 in a downward direction.

Printer housing 24 and solenoid block 26 are affixed to end plate 58 by screws 292. Plate 58 and lifting bar 50 are coupled by way of a pivot pin 294 on which the plate is rotatable relative to bar 50. The angle of relative rotation, however, is limited by a second pin 296 which extends from plate 58 and projects through an oversized aperture in .bar 50. If it were not for the latter, the plate 58 would rotate under the weight of the solenoid block 26 and strike the paper and platen when the shaft 62 was rotated sufficiently to raise the print housing legs 280a, 28012 away from platen 16. It will be noted that the lower edge of plate 58 is higher than the bottom faces of legs 280a, 28012.

The physical relationship of the components described above is such that there is a slight clearance between cam roller 54 and the chamfered edge of bar 50 when no paper is present at the print station and legs 280a, 28Gb are resting on platen '16. Bias spring 290 exerts a down ward force on bar 50 which, in turn exerts a downward force on pin 294. This pin is centrally located relative to legs 280a and 280b, whereby the force exerted on each leg is equal. Since the end plate 58, and hence housing 26 and solenoid block 26, are rotatable on pin 294, the structure is self equalizing insofar as the pressure on legs 280a and 28% is concerned.

When a sheaf of papers and carbons is present between platen 16 and the print head, the right end of bar 50 will be at a position elevated from that shown in FIGURE 5.

However, the bias force exerted by spring 2% will exert a downward force on pin 294. Plate 58, and hence housing 24 and solenoid block 26 will rotate under this force to assume a position in which the bottom faces of legs 280a and 28% are parallel to the platen, and in which the clamping forces exerted by the legs are equal. The low rate spring 2% allows the clamping force to be independent of the number of papers and carbons.

The printer bar structures (FIGURE 3) are so suspended that they are free to pivot under their own weight into light contact with the top sheet of the sheaf when the solenoids are deenergized. Since the faces of legs 280a, 28% also are in contact with the sheaf, and since the printer bar supporting pivots and the legs are part of a unitary structure, it follows that the rest positions of the printer bar structures relative to the side 'walls and legs is independent of the number of interleaved papers and carbons. Stated in another way, the printing surfaces of the printer bar structures relative to the platen 16 varies according to the thickness of the sheaf; however, the air gaps of the printer solenoids, once set, remain constant and are independent of the number of copies being printed. This is of great importance, as discussed previously, because the stroke of the printer bars may be about 0.005 inch or less.

What is claimed is:

1. In a printer, the combination of:

a document path;

a platen on one side of said path having a fiat surface adjacent said path;

a print head located on the opposite side of said path and including first and second spaced side members extending in a direction along the width of said path, and a plurality of printer elements disposed between said side members;

said first and second side members each terminating at one end in an elongated, flat surface facing said document path, the flat surfaces of the respective side members lying in the same plane; and

means biasing said print head toward said platen with the fiat surfaces of said side members pressed firmly against said flat surface of said platen.

2. In a print station having a document receiving path, and a platen disposed on one side of said path and having a fiat surface against which the documents in said station rest, the combination of;

a print head located on the opposite side of said path and including first and second side members extendin a direction along the width of said path;

a plurality of printer element in said head between said side members; 7

each of said side members having an elongated flat bottom surface facing said path, the flat bottom surfaces of the side members lying in the same plane; and

means biasing said print head toward said platen withthe fiat surfaces of said member pressing the documents in said path firmly against the platen to squeeze out any air between said documents.

3. The combination as claimed in claim 2, wherein the flat surfaces of said legs are parallel to the flat surface of said platen when said documents are pressed against said platen, and including means for selectively moving said print head away from said platen to allow document advance along said path.

4. In a printer, having a document path for receiving documents to be printed, the combination of:

a print head on one side of said path having first and second elongated side members extending in a direction along the width of said path, and a plurality of elongated printer bar structures stacked side-by-side between said side members and held in stacked array thereby, the array of printer bar structures having a thickness T in a direction normal to the lengths thereof;

a platen located on the other side of said path and having an elongated channel underlying the array of printer bar structures and having a width W T;

a printer anvil movable in said channel;

each of said side members being terminated in an elongated leg adjacent aid path, the faces of said legs adjacent said path being flat and lying in the same plane, and the legs being spaced apart a distance D, where D W; and

means biasing said print head toward said platen so that the faces of said legs press the documents firmly against said platen on both sides of said channel to squeeze out the air between said documents.

5. The combination as claimed in claim 4, wherein each printer :bar structure has a fiat printing surface parallel to the faces of said legs and facing said document path.

6. The combination as claimed in claim 5, wherein said printer bar structures and said side members are substantially vertical, and wherein said printer bar structures are suspended for movement under their own weight into contact with the top document in said path.

7. The combination as claimed in claim 5, including means mounting said print head for limited rotation about an axis parallel to said side members, whereby said print 10 head can assume a position in which the faces of said legs apply equal force on said documents.

8. The combination as claimed in claim 5. wherein the bias force on said print head is applied substantially midway between said elongated legs.

References Cited UNITED STATES PATENTS ROBERT E. PULFREY, Primary Examiner. E. S. BURR, Assistant Examiner. 

1. IN A PRINTER, THE COMBINATION OF: A DOCUMENT PATH; A PLATEN ON ONE SIDE OF SAID PATH HAVING A FLAT SURFACE ADJACENT SAID PATH; A PRINT HEAD LOCATED ON THE OPPOSITE SIDE OF SAID PATH AND INCLUDING FIRST AND SECOND SPACED SIDE MEMBERS EXTENDING IN A DIRECTION ALONG THE WIDTH OF SAID PATH, AND A PLURALITY OF PRINTER ELEMENTS DISPOSED BETWEEN SAID SIDE MEMBERS; SAID FIRST AND SECOND SIDE MEMBERS EACH TERMINATING AT ONE END IN AN ELONGATED, FLAT SURFACE FACING SAID DOCUMENT PATH, THE FLAT SURFACES OF THE RESPECTIVE SIDE MEMBER LYING IN THE SAME PLANE; AND MEANS BIASING SAID PRINT HEAD TOWARD SAID PLATEN WITH THE FLAT SURFACES OF SAID SIDE MEMBERS PRESSED FIRMLY AGAINST SAID FLAT SURFACE OF SAID PLATEN. 